Siltation
Encyclopedia
Siltation is the pollution of water by fine particulate terrestrial clastic material, with a particle size dominated by silt
or clay
. It refers both to the increased concentration of suspended sediments, and to the increased accumulation (temporary or permanent) of fine sediments on bottoms where they are undesirable. Siltation is most often caused by soil erosion or sediment spill.
Sometimes siltation is called sediment pollution, although that is an unfortunate term since it is ambiguous, and can also be used to refer to a chemical contamination of sediments accumulated on the bottom, or pollutants bound to sediment particles. Siltation is the preferred term for being unambigiuous, even if not entirely stringent since it also includes other particle sizes than silt.
In rural areas the erosion source is typically soil degradation due to intensive or inadequate agricultural practices, leading to soil erosion, especially in fine-grained soils such as loess
. The result will be an increased amount of silt and clay in the water bodies that drain the area. In urban areas the erosion source is typically construction activities, since this involves clearing the original land-covering vegetation and temporarily creating something akin to an urban desert from which fines are easily washed out during rainstorms.
In water the main pollution source is sediment spill from dredging, from the transportation of dredged material on barges, and the deposition of dredged material in or near water. Such deposition may be made to get rid of unwanted material, such as the offshore dumping of material dredged from harbours and navigation channels. The deposition may also have as purpose to build up the coastline, artificial islands, or for beach replenishment.
Climate change
also affect siltation rates.
is in transport
in suspension
, it acts as a pollutant for those who require clean water. This includes uses such as for cooling or in industrial processes, and it includes aquatic life that are sensitive to suspended material in the water. While nekton
have been found to avoid spill plumes in the water (e.g. the environmental monitoring project during the building of the Øresund Bridge), filtering benthic organisms have no way of escape. Among the most sensitive organisms are coral
polyps. Generally speaking, hard bottom communities and mussel banks (including oysters) are more sensitive to siltation than sand and mud bottoms. Unlike in the sea, in a stream the plume will cover the entire channel, except possibly for backwaters, why also fish will be directly affected in most cases.
Siltation can also affect navigation channels, or irrigation channels. It refers to the undesired accumulation of sediments in channels intended for vessels, or for distributing water.
. Example: 50 mg/L times 30 m3/s gives 1.5 kg/s.
Also sediment spill is better measured in transport than at the source. The sediment transport in open water is estimated by measuring the turbidity
, correlating turbidity to sediment concentration (using a regression developed from water samples that are filtered, dried, and weighed), multiplying the concentration with the discharge as above, and integrating over the entire plume. To distinguish the spill contribution, the background turbidity is subtracted from the spill plume turbidity. Since the spill plume in open water varies in space and time, an integration over the entire plume is required, and repeated many times to get acceptably low uncertainty in the results. These measurements are made close to the source, in the order of a few hundred meters.
Anything beyond a work area buffer zone for sediment spill is considered the potential impact area. In the open sea the impact of concern is almost exclusively with the sessile
bottom communities, since empirical data show that fish effectively avoid the impacted area. The siltation chiefly affects the bottom community in two ways: The suspended sediment may interfere with the food gathering of filtering organisms, and the sediment accumulation on the bottom may bury organisms to the point that they starve or even die. Only if the concentration is extreme will it decrease the light level sufficiently for impacting primary productivity. An accumulation of as little as 1 mm may kill coral polyps.
While the effect of the siltation on the biota (once the harm is already done) can be studied by repeated inspection of selected test plots, the magnitude of the siltation process in the impact area may be measured directly by monitoring in real time. Parameters to measure are sediment accumulation, turbidity at the level of the filtering biota, and optionally incident light.
Siltation of the magnitude that it affects shipping can also be monitored by repeated bathymetric surveys
.
). In urban areas the defense is to keep land uncovered for as short a time as possible during construction, and to use silt screens to prevent the sediment from getting released in water bodies. During dredging the spill can be minimized but not eliminated completely through the way the dredger is designed and operated. If the material is deposited on land, efficient sedimentation basins can be constructed. If it is dumped in relatively deep water there will be a significant spill during dumping, but not thereafter, and the spill that does arise will have minimal impact if there are only fine-sediment bottoms nearby.
One of the most difficult conflicts of interest to resolve, as regards siltation mitigation, is perhaps beach nourishment
. When sediments are placed on or near beaches in order to replenish an eroding beach, any fines in the material will continue to be washed out for as long as the sand is being reworked. Given that all replenished beaches are eroding (or else they would not need replenishment), they will contribute to nearshore siltation almost for as long as it takes to erode away what was added, albeit with somewhat decreasing intensity over time. Since this leakage is detrimental to coral reefs, the practice leads to a direct conflict between the public interest of saving beaches, and preserving any nearshore coral reefs. To minimize the conflict, beach replenishment should only be done with sand not containing any silt or clay fractions. In practice the sand is often taken from offshore borrowing areas, and since the proportion of fines in sediments typically increases in the offshore directioon, the deposited sand will inevitably contain a significant percentage of siltation-contributing fines.
It is desirable to minimize the siltation of irrigation channels by hydrologic design, the objective being not to create zones with falling sediment transport capacity, as that is conducive to sedimentation. Once sedimentation has occurred, in irrigation or navigation channels, dredging is often the only remedy.
Silt
Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...
or clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
. It refers both to the increased concentration of suspended sediments, and to the increased accumulation (temporary or permanent) of fine sediments on bottoms where they are undesirable. Siltation is most often caused by soil erosion or sediment spill.
Sometimes siltation is called sediment pollution, although that is an unfortunate term since it is ambiguous, and can also be used to refer to a chemical contamination of sediments accumulated on the bottom, or pollutants bound to sediment particles. Siltation is the preferred term for being unambigiuous, even if not entirely stringent since it also includes other particle sizes than silt.
Causes
The origin of the increased sediment transport into an area may be erosion on land, or activities in the water.In rural areas the erosion source is typically soil degradation due to intensive or inadequate agricultural practices, leading to soil erosion, especially in fine-grained soils such as loess
Loess
Loess is an aeolian sediment formed by the accumulation of wind-blown silt, typically in the 20–50 micrometre size range, twenty percent or less clay and the balance equal parts sand and silt that are loosely cemented by calcium carbonate...
. The result will be an increased amount of silt and clay in the water bodies that drain the area. In urban areas the erosion source is typically construction activities, since this involves clearing the original land-covering vegetation and temporarily creating something akin to an urban desert from which fines are easily washed out during rainstorms.
In water the main pollution source is sediment spill from dredging, from the transportation of dredged material on barges, and the deposition of dredged material in or near water. Such deposition may be made to get rid of unwanted material, such as the offshore dumping of material dredged from harbours and navigation channels. The deposition may also have as purpose to build up the coastline, artificial islands, or for beach replenishment.
Climate change
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...
also affect siltation rates.
Vulnerabilities
While the sedimentSediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
is in transport
Sediment transport
Sediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained...
in suspension
Suspension (chemistry)
In chemistry, a suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometer. The internal phase is dispersed throughout the external phase through mechanical agitation, with the use of certain...
, it acts as a pollutant for those who require clean water. This includes uses such as for cooling or in industrial processes, and it includes aquatic life that are sensitive to suspended material in the water. While nekton
Nekton
Nekton refers to the aggregate of actively swimming aquatic organisms in a body of water able to move independently of water currents....
have been found to avoid spill plumes in the water (e.g. the environmental monitoring project during the building of the Øresund Bridge), filtering benthic organisms have no way of escape. Among the most sensitive organisms are coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
polyps. Generally speaking, hard bottom communities and mussel banks (including oysters) are more sensitive to siltation than sand and mud bottoms. Unlike in the sea, in a stream the plume will cover the entire channel, except possibly for backwaters, why also fish will be directly affected in most cases.
Siltation can also affect navigation channels, or irrigation channels. It refers to the undesired accumulation of sediments in channels intended for vessels, or for distributing water.
Measurement and monitoring
One may distinguish between measurements at the source, during transport, and within the affected area. Source measurements of erosion may be very difficult, since the lost material may be a fraction of a millimeter per year. Therefore the approach taken is typically to measure the sediment in transport in the stream, by measuring the sediment concentration and multiplying that with the dischargeDischarge (hydrology)
In hydrology, discharge is the volume rate of water flow, including any suspended solids , dissolved chemical species and/or biologic material , which is transported through a given cross-sectional area...
. Example: 50 mg/L times 30 m3/s gives 1.5 kg/s.
Also sediment spill is better measured in transport than at the source. The sediment transport in open water is estimated by measuring the turbidity
Turbidity
Turbidity is the cloudiness or haziness of a fluid caused by individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality....
, correlating turbidity to sediment concentration (using a regression developed from water samples that are filtered, dried, and weighed), multiplying the concentration with the discharge as above, and integrating over the entire plume. To distinguish the spill contribution, the background turbidity is subtracted from the spill plume turbidity. Since the spill plume in open water varies in space and time, an integration over the entire plume is required, and repeated many times to get acceptably low uncertainty in the results. These measurements are made close to the source, in the order of a few hundred meters.
Anything beyond a work area buffer zone for sediment spill is considered the potential impact area. In the open sea the impact of concern is almost exclusively with the sessile
Sessility (limnology)
In limnology, sessility is that quality of an organism which rests unsupported directly on a base, either attached or unattached to a substrate. It is a characteristic of vegetation which is anchored to the benthic environment. There are two families of sessile rotifers: Flosculariidae and...
bottom communities, since empirical data show that fish effectively avoid the impacted area. The siltation chiefly affects the bottom community in two ways: The suspended sediment may interfere with the food gathering of filtering organisms, and the sediment accumulation on the bottom may bury organisms to the point that they starve or even die. Only if the concentration is extreme will it decrease the light level sufficiently for impacting primary productivity. An accumulation of as little as 1 mm may kill coral polyps.
While the effect of the siltation on the biota (once the harm is already done) can be studied by repeated inspection of selected test plots, the magnitude of the siltation process in the impact area may be measured directly by monitoring in real time. Parameters to measure are sediment accumulation, turbidity at the level of the filtering biota, and optionally incident light.
Siltation of the magnitude that it affects shipping can also be monitored by repeated bathymetric surveys
Bathymetry
Bathymetry is the study of underwater depth of lake or ocean floors. In other words, bathymetry is the underwater equivalent to hypsometry. The name comes from Greek βαθύς , "deep", and μέτρον , "measure"...
.
Mitigation
In rural areas the first line of defense is to maintain land cover and prevent soil erosion in the first place. The second line of defense is to trap the material before it reaches the stream network (known as sediment controlSediment control
A sediment control is a practice or device designed to keep eroded soil on a construction site, so that it does not wash off and cause water pollution to a nearby stream, river, lake, or bay...
). In urban areas the defense is to keep land uncovered for as short a time as possible during construction, and to use silt screens to prevent the sediment from getting released in water bodies. During dredging the spill can be minimized but not eliminated completely through the way the dredger is designed and operated. If the material is deposited on land, efficient sedimentation basins can be constructed. If it is dumped in relatively deep water there will be a significant spill during dumping, but not thereafter, and the spill that does arise will have minimal impact if there are only fine-sediment bottoms nearby.
One of the most difficult conflicts of interest to resolve, as regards siltation mitigation, is perhaps beach nourishment
Beach nourishment
Beach nourishment— also referred to as beach replenishment—describes a process by which sediment lost through longshore drift or erosion is replaced from sources outside of the eroding beach...
. When sediments are placed on or near beaches in order to replenish an eroding beach, any fines in the material will continue to be washed out for as long as the sand is being reworked. Given that all replenished beaches are eroding (or else they would not need replenishment), they will contribute to nearshore siltation almost for as long as it takes to erode away what was added, albeit with somewhat decreasing intensity over time. Since this leakage is detrimental to coral reefs, the practice leads to a direct conflict between the public interest of saving beaches, and preserving any nearshore coral reefs. To minimize the conflict, beach replenishment should only be done with sand not containing any silt or clay fractions. In practice the sand is often taken from offshore borrowing areas, and since the proportion of fines in sediments typically increases in the offshore directioon, the deposited sand will inevitably contain a significant percentage of siltation-contributing fines.
It is desirable to minimize the siltation of irrigation channels by hydrologic design, the objective being not to create zones with falling sediment transport capacity, as that is conducive to sedimentation. Once sedimentation has occurred, in irrigation or navigation channels, dredging is often the only remedy.